1. Field of the Invention
The present invention relates to a perovskite oxide, a process for producing the perovskite oxide, a ferroelectric compound containing the perovskite oxide, a piezoelectric body, a piezoelectric device using the piezoelectric body, and a liquid discharge device using the piezoelectric body.
2. Description of the Related Art
Currently, piezoelectric devices constituted by a piezoelectric body and electrodes are used as, for example, actuators installed in inkjet recording heads. In the piezoelectric devices, the piezoelectric body expands and contracts according to increase and decrease in the strength of an electric field applied from the electrodes to the piezoelectric body in a predetermined direction. For example, perovskite oxides such as PZT (lead titanate zirconate) are known as materials suitable for the piezoelectric body. Such materials are ferroelectric materials which exhibit spontaneous polarization even when no electric field is applied. The piezoelectric materials are reported to exhibit high piezoelectric performance at and near the morphotropic phase boundary (MPB).
PZT is a solid solution of PbTiO3 (PT) and PbZrO3 (PZ). FIG. 14 is a phase diagram of PZT with respect to the temperature and the mole fraction of titanium (i.e., the mole fraction of PbTiO3) in PZT. The phase diagram of FIG. 14 is cited from “Landolt-Bornstein: Numerical Data and Functional Relationships in Science and Technology, New Series,” Group III: Crystal and Solid State Physics, Vol. 16, edited by K. H. Hellwege and A. M. Hellwege, Springer-Verlag Berlin-Heidelberg-New York (1981) p. 426 & FIG. 728. In FIG. 14, FT denotes the tetragonal phase, and FR denotes the rhombohedral phase.
PZT tends to form tetragonal crystals when the Ti composition is high, and rhombohedral crystals when the Zr composition is high. When the molar compositions of Ti and Zr are approximately identical, the MPB composition is achieved. For example, the mole ratio of Zr to Ti of 52/48, which is near to the MPB composition, is preferable. The textbooks on the piezoelectric ceramic materials teach that the crystal structures become unstable and the piezoelectric performance becomes highest at and near the MPB. Conventionally, PZT has been reported to form pseudocubic crystals at and near the MPB. However, details of the nanostructure of PZT are unknown.
In the above circumstances, the Japanese Unexamined Patent Publication No. 2006-036578 (hereinafter referred to as JP2006-036578) reports that sintered bodies of PZT-based ceramic materials such as Pb(Ti, Zr, Nb)O3 are formed of two-phase mixed crystals of tetragonal crystals and rhombohedral crystals at and near the MPB. (See, for example, claim 9 in JP2006-036578.) Further, JP2006-036578 discloses that it is possible to desirably design the composition on the basis of the relationship between the piezoelectric coefficient and the phase fractions of the tetragonal and rhombohedral phases. (See, for example, Table 1, FIG. 4, and the paragraph 0027 in JP2006-036578.)
In addition, S. Yokoyama et al., “Compositional Dependence of Electrical Properties of Highly (100)-/(001)-Oriented Pb(Zr, Ti) O3 Thick Films Prepared on Si Substrates by Metalorganic Chemical Vapor Deposition”, Japanese Journal of Applied Physics, Vol. 42, pp. 5922-5926, 2003 also report that PZT films are formed of two-phase mixed crystals of tetragonal crystals and rhombohedral crystals at and near the MPB. (See, for example, FIG. 2(b) in the Yokoyama reference.)
However, in the conventional piezoelectric devices of a first type, the piezoelectric effect of expansion of a ferroelectric body in the direction of the spontaneous polarization is generally utilized by applying an electric field to the ferroelectric body along the direction of the spontaneous polarization. That is, it has been conventionally considered important to design the piezoelectric materials so that the electric field is applied along the direction of the spontaneous polarization. Nevertheless, in the case where only the piezoelectric effect of expansion in the direction of the spontaneous polarization is utilized, the amount of displacement is limited, although greater displacement is currently demanded.
The Japanese Patent No. 3568107 (hereinafter referred to as JP3568107) proposes the conventional piezoelectric devices of a second type, in which application of an electric field induces phase transition in a piezoelectric body. JP3568107 discloses a piezoelectric device constituted by a phase-transition film, electrodes, and a heating body, where the heating body adjusts the temperature of the phase-transition film to a level near to the Curie point Tc. (See claim 1 in JP3568107.) JP3568107 refers to use, as the phase-transition film, of a film in which transition occurs between a tetragonal phase and a rhombohedral phase or between a cubic phase and a tetragonal or rhombohedral phase. (See claim 2 in JP3568107.) Further, JP3568107 reports that the conventional piezoelectric devices of the second type disclosed in JP3568107 can achieve greater displacement than the conventional piezoelectric devices of the first type because both of the piezoelectric effect of the ferroelectric material and the change in the crystal structure associated with the phase transition contribute to the displacement.
As explained above, although PZT has been conventionally reported to form pseudocubic crystals at and near the MPB, JP2006-036578 and the Yokoyama reference report that PZT-based ceramic materials form a two-phase mixed-crystal structure containing a tetragonal phase and a rhombohedral phase at and near the MPB. However, many aspects of the piezoelectric mechanism and the crystal structure at and near the MPB are still unknown.
In addition, according to the technique disclosed in JP2006-036578, it is necessary to prepare samples of perovskite oxides each constituted by a plurality of predetermined elements with different mole fractions, obtain the phase fractions of the tetragonal and rhombohedral phases in each sample by X-ray diffraction and Rietveld analysis, obtain the piezoelectric coefficient of each sample, and determine the composition on the basis of the relationship between the obtained phase fractions and the piezoelectric coefficient. However, according to the above technique, it is necessary to search for desirable composition by performing an experiment every time the constituent elements of the sample are changed, so that the material design cannot be efficiently made by the technique.
Incidentally, as mentioned before, JP3568107 refers to the use, as the phase-transition film, of films in which phase transition occurs between a tetragonal system and a rhombohedral system or between a cubic system and a tetragonal or rhombohedral system. However, the piezoelectric devices disclosed in JP3568107 are assumed to be used in the vicinity of the Curie point Tc. Since the Curie point Tc corresponds to the phase-transition temperature between the ferroelectric phase and the paraelectric phase, the phase transition between the tetragonal phase and the rhombohedral phase occurs in no film when the film is used in the vicinity of the Curie point Tc. That is, the piezoelectric devices disclosed in JP3568107 cannot utilize the phase transition other than the phase transition between the ferroelectric phase and the paraelectric phase. In addition, since spontaneous polarization does not occur in the paraelectric material, the piezoelectric devices disclosed in JP3568107 do not exhibit the piezoelectric effect of expansion in the direction of the polarization in response to application of an electric field after the phase transition.
The present inventor and colleagues belonging to the present assignee have proposed in the International Patent Application Publication No. WO2007/034903 (which is hereinafter referred to as WO2007/034903) a piezoelectric device using a piezoelectric body which contains regions in a first ferroelectric phase having crystal orientation. In the piezoelectric body, the phase of at least a portion of the above regions transitions from the first ferroelectric phase corresponding to a first crystal system to a second ferroelectric phase corresponding to a second crystal system different from the first crystal system when an electric field is applied to the piezoelectric body.
In the above piezoelectric device, it is possible to achieve a volume change caused by a change in the crystal structure associated with the phase transition from the first ferroelectric phase. In addition, since the piezoelectric effect works in both of the first ferroelectric phase (before the phase transition) and the second ferroelectric phase (after the phase transition), the piezoelectric device disclosed in WO2007/034903 exhibits greater displacement than the piezoelectric devices disclosed in JP3568107 when the electric field is applied to the piezoelectric body.
Further, WO2007/034903 reports that the engineered-domain effect and the like increase the distortion amount (displacement) when the direction along which the electric field is applied to the piezoelectric body is different from the orientation of the spontaneous polarization axis in the ferroelectric phase before the phase transition, and is preferably approximately identical to the orientation of the spontaneous polarization axis in the ferroelectric phase after the phase transition.